Idiot, fool.
The best-known Japanese insult abroad and genuinely common at home. Its weight swings hard with tone: teasing between friends or partners, or a real insult said coldly. In Kansai the equivalent is アホ (aho), and the two regions each find the other's word harsher.
In Tokyo, バカ is lighter and アホ is harsh. In Osaka it is exactly the reverse. Same words, inverted weight.
Reading a definition isn't the same as saying it to someone. Lingo Kaiava is a voice tutor set to Japanese — it uses バカ the way locals do, corrects you in the moment, and never "fixes" the way real people actually talk into textbook Japanese.
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